linux-kernel - Re: [PATCH] cleanup: Fix unused guard error function with DEFINE_CLASS_IS_COND

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Message-ID: <20250813141346.GM4067720@noisy.programming.kicks-ass.net>
Date: Wed, 13 Aug 2025 16:13:46 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Dan Williams <dan.j.williams@...el.com>
Cc: dave.jiang@...el.com, linux-kernel@...r.kernel.org,
	linux-cxl@...r.kernel.org, Nathan Chancellor <nathan@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Lechner <dlechner@...libre.com>,
	Jonathan Cameron <jonathan.cameron@...wei.com>,
	Andy Shevchenko <andriy.shevchenko@...el.com>
Subject: Re: [PATCH] cleanup: Fix unused guard error function with
 DEFINE_CLASS_IS_COND_GUARD

On Mon, Aug 04, 2025 at 03:09:54PM -0700, Dan Williams wrote:
> Andy reports that the "lock_timer" scheme in kernel/time/posix-timers.c,
> with its custom usage of DEFINE_CLASS_IS_COND_GUARD(), results in:
> 
> kernel/time/posix-timers.c:89:1: error: unused function 'class_lock_timer_lock_err' [-Werror,-Wunused-function]
>    89 | DEFINE_CLASS_IS_COND_GUARD(lock_timer);
> 
> ...with a clang W=1 build. Per Nathan, clang catches unused "static inline"
> functions in C files since commit 6863f5643dd7 ("kbuild: allow Clang to
> find unused static inline functions for W=1 build").

I so loathe that warning :/

> There are 2 ways to solve this, either mark the class_##_lock_err()
> function as __maybe_unused, or make sure class_##_lock_err() *is* used /
> gets called to disposition the lock status.
> 
> At present __lock_timer() only indicates failure with a NULL __guard_ptr().
> However, one could imagine that __lock_timer(), or some other custom
> conditional locking primitive, wants to pass an ERR_PTR() to indicate the
> reason for the lock acquisition failure.
> 
> Update __scoped_cond_guard() to check for ERR_PTR() in addition to NULL
> @scope values. This allows __lock_timer(), or another open coded
> DEFINE_CLASS_IS_COND_GUARD() user, to switch to passing an ERR_PTR() in the
> future. In the meantime, this just silences the warning.
> 
> Cc: Nathan Chancellor <nathan@...nel.org>
> Cc: "Peter Zijlstra (Intel)" <peterz@...radead.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Dave Jiang <dave.jiang@...el.com>
> Cc: David Lechner <dlechner@...libre.com>
> Cc: Jonathan Cameron <jonathan.cameron@...wei.com>
> Reported-by: Andy Shevchenko <andriy.shevchenko@...el.com>
> Closes: http://lore.kernel.org/aIo18KZpmKuR4hVZ@black.igk.intel.com
> Fixes: 857d18f23ab1 ("cleanup: Introduce ACQUIRE() and ACQUIRE_ERR() for conditional locks")
> Signed-off-by: Dan Williams <dan.j.williams@...el.com>
> ---
> Dave, I am sending this to you to take upstream since the warning came
> in through the CXL tree. If anyone else wants to take it just holler.
> 
>  include/linux/cleanup.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/cleanup.h b/include/linux/cleanup.h
> index 4eb83dd71cfe..d8e7d1e5561b 100644
> --- a/include/linux/cleanup.h
> +++ b/include/linux/cleanup.h
> @@ -423,7 +423,8 @@ _label:									\
>  
>  #define __scoped_cond_guard(_name, _fail, _label, args...)		\
>  	for (CLASS(_name, scope)(args); true; ({ goto _label; }))	\
> -		if (!__guard_ptr(_name)(&scope)) {			\
> +		if (!__guard_ptr(_name)(&scope) ||			\
> +		     __guard_err(_name)(&scope)) {			\
>  			BUILD_BUG_ON(!__is_cond_ptr(_name));		\
>  			_fail;						\
>  _label:									\

What does this do for code generation ?